1. Field of the Invention
This invention relates to elements bearing coatings for preventing the deposition of carbon, or coke, on fuel wetted surfaces located in high temperature zones of gas turbine engines. Coke deposition is an undesirable side effect caused by the catalytic-thermal degradation of hydrocarbon fuels during their consumption in gas turbine engines. Such deposition leads to performance loss, reduced heat transfer efficiencies, increased pressure drops, costly decoking procedures, and increased rates of material corrosion and erosion. The metals most prone to catalyze coke deposition are those metals commonly found in the alloys utilized in components exposed to high temperature, fuel wetted environments of gas turbine engines, typically found in jet engines in the combustor and afterburner fuel delivery systems.
2. Description of the Prior Art
Carburization, or the formation of coke deposits, has been noted particularly in high temperature environments where carbon containing fluids come in contact with metals or metal alloys. Exemplary of such environments are high temperature reactors, such as refinery crackers, thermal crackers, distillation units for petroleum feedstock, and gas turbine components. Conventional methods used to reduce coke formation and carburization in steam cracking operations involve the steam pre-treatment of the surface to promote formation of a protective oxide skin. The surface may then be further protected by the deposition of a high temperature, stable, non-volatile metal oxide on the pre-oxidized substrate surface by thermal decomposition from the vapor phase of a volatile compound of the metal.
While the chemical vapor deposition of an alkoxysilane has been demonstrated to reduce the rate of coke formation in the pyrolysis section of an ethylene steam cracker by formation of an amorphous silica film on the internal surfaces of high alloy steel tubing at 700.degree. to 800.degree. C., no one to date has solved the problem of coke deposition on fuel contacting hardware in gas turbine engines.
Alumina coatings have been previously applied to a large number of substrates for various purposes, but not, to our knowledge, for the prevention of coke deposition on fuel contacting elements in gas turbines, prior to the teachings of U.S. patent application Ser. Nos. 07/811,356 and 07/811,359 of Edwards, filed Dec. 20, 1991, and incorporated herein by reference. For example, flame sprayed coatings of alumina have been applied to foundry molds, but lacked adherence due to thermal shock. In U.S. Pat. No. 2,903,375, Peras attempted to overcome this problem by applying layered coatings of cermets containing alumina and chromium. Montgomery et at, in U.S. Pat. No. 2,775,531, suggest the application of aluminum-alumina cermets to metal substrates by flame spraying and sintering to provide high temperature oxidation resistance and thermal insulation. In U.S. Pat. No. 3,839,618, Muehlberger teaches spray coating stainless steel with a dielectric layer of alumina. Hecht, in U.S. Pat. No. 4,034,142, teaches the protection of nickel and cobalt superalloy articles at elevated temperatures by the formation of a coating having an external continuous layer composed predominately of alumina, which reduces oxidation and corrosion.
As indicated, various processes have been used to deposit ceramic materials such as alumina upon a substrate. These include the application of glazes, enamels, and coatings; hot-pressing materials at elevated pressure and temperature; and, vapor deposition processes such as evaporation, cathodic sputtering, chemical vapor deposition, flame spraying, and plasma spraying. In addition, electrophoresis has been attempted, as have other specialized techniques, with limited success in application.
For example, the enamelling industry has used the electrodeposition of ceramic materials for some time. In the application of a ceramic coating by this technique, a ceramic material is milled or ground to a small particulate or powder size, placed into suspension, and electrophoretically deposited on the substrate. Another traditional method is the deposition of a ceramic coating from a slurry made up of a powder in suspension, usually in an aqueous medium. A major problem with these techniques is that powder particle sizes below about 2 microns were difficult to obtain, thus limiting the quality of coatings produced.
Sol-gel technology has recently evolved as a source of very fine sub-micron ceramic particles of great uniformity. Such sol-gel technology comprises essentially the preparation of ceramics by low temperature hydrolysis and peptization of metal oxide precursors in solution, rather than by the sintering of compressed powders at high temperatures.
In the prior art, much attention has been given to the preparation of sols of metal oxides (actually metal hydroxides, in most cases) by hydrolysis and peptization of the corresponding metal alkoxide, such as aluminum sec-butoxide [Al(OC.sub.4 H.sub.9).sub.3 ], in water, with an acid peptizer such as hydrochloric acid, acetic acid, nitric acid, and the like. The hydrolysis of aluminum alkoxides is discussed in an article entitled "Alumina Sol Preparation from Alkoxides" by Yoldas, in American Ceramic Society Bulletin Vol. 54, No. 3 (1975), pages 289-290. This article teaches the hydrolysis of aluminum alkoxide precursor with a mole ratio of water:precursor of 100:1, followed by peptization at 90.degree. with 0.07 moles of acid per mole of precursor. After gelling and drying, the dried gel is calcined to form alumina powder.
In U.S. Pat. No. 4,532,072, of Segal, an alumina sol is prepared by mixing cold water and aluminum alkoxide in stoichiometric ratio, allowing them to react to form a peptizable aluminum hydrate, and peptizing the hydrate with a peptizing agent in an aqueous medium to produce a sol of an aluminum compound.
In Clark et al, U.S. Pat. No. 4,801,399, a method for obtaining a metal oxide sol is taught whereby a metal alkoxide is hydrolysed in the presence of an excess of aqueous medium, and peptized in the presence of a metal salt, such as a nitrate, so as to obtain a particle size in the sol between 0.0001 micron and 10 microns.
In Clark et al, U.S. Pat. No. 4,921,731, a method is taught for ceramic coating a substrate by thermophoresis of sols of the type prepared by the method of U.S. Pat. No. 4,801,399. In addition, Clark et al, in abandoned U.S. patent application Ser. No. 06/841,089, filed Feb. 25, 1986, teach formation of ceramic coatings on a substrate, including filaments, ribbons, and wires, by electrophoresis of such sols. However, the examples of this application indicate that the coatings obtained using electrophoresis were uneven, cracked, and contained bubbles, and often peeled, flaked off, and/or pulled apart.